1. Field of the Invention
The present invention relates to a method of producing a lithium ion secondary battery comprising a nonaqueous electrolytic solution and more particularly to a method of producing and the structure of a safe lithium ion battery having a high charge-discharge efficiency comprising a low fluidity or gelled electrolytic solution.
2. Description of the Related Art
Portable electronic apparatus have found a very great demand for reduced size and weight. The accomplishment of the demand greatly depends on the enhancement of the properties of the battery to be mounted in these portable electronic apparatus. In order to meet this demand, the development and improvement of various batteries are under way. In particular, a lithium battery is a secondary battery which can realize the highest voltage, energy density and load resistance in the existing batteries. The improvement of lithium batteries is still under way.
FIG. 1 is a schematic sectional view illustrating the structure of an ordinary lithium ion secondary battery which has been put into practical use. The lithium ion secondary battery comprises as essential constituents a positive electrode 1, a negative electrode 2, and an ionically-conducting layer 3 provided interposed therebetween. In this lithium ion secondary battery, as the positive electrode 1 there is used a plate-like material prepared by applying a mixture of an active positive electrode material powder 1a such as lithium-cobalt composite oxide, an electrically-conducting powder 1b and a binder resin to a positive electrode collector 1c. Similarly, as the negative electrode 2 there is used a plate-like material prepared by applying a mixture of a carbon-based negative electrode active material powder 2a and a binder resin to a negative electrode collector 2c made of copper. As the ionically-conducting layer 3 there is used a separator made of a porous film of polyethylene or polypropylene filled with a nonaqueous electrolytic solution containing lithium ion. The battery structure of the present example comprises a single electrode laminate 4 having a separator laminated with an electrode.
The lithium ion battery comprising such a nonaqueous electrolytic solution is liable to rise in the danger of sparking, heat generation, etc. due to internal or external shortcircuiting caused by the rise in battery capacity. The rise in battery capacity faces a great apprehension that the battery might ignite. The elimination of this danger can be effectively accomplished by the reduction of the fluidity of the electrolytic solution. However, since the lithium ion battery comprises a porous electrode formed by coagulating a particulate active material, it is very difficult for an electrolytic solution having a reduced fluidity to fill thoroughly microvoids in the electrode. On the other hand, it is necessary that the microvoids in the electrode be filled with an electrolytic solution to improve the battery properties.
Further, gelled electrolytes have been of interest and under extensive study for practical use from the standpoint of the reduction of the thickness of batteries. However, the gelled electrolytes, too, cannot be easily injected into the electrode. It is thus very difficult for the gelled electrolytes to fill thoroughly voids in the electrode. Batteries comprising a gelled electrolyte are disclosed in U.S. Pat. No. 5,460,904, "Nikkei Microdevice", Nikkei BP, August 1996, page 136, etc.
As mentioned above, it is not easy for any electrolytic solution to fill thoroughly microvoids in the electrode. Thus, the production of such a battery faces a problem that it is difficult to fill thoroughly voids in the electrode. This makes it impossible to provide a safe lithium ion secondary battery having a high charge-discharge efficiency.